Method of preparing phosphine



Nov. 5, 1963 GORDON 3,109,795

METHOD OF PREPARING PHOSPHINE Filed July 2'7, 1960 United States Patent ()fiice 3 ,109,795 Patented Nov. 5., 1963 3,109,795 METHOD OF PREPARING PHOSPHINE Irving Gordon, Niagara Falls, N.Y., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed July 27, 1960, Ser. No. 45,669 11 Claims. (Cl. 204-101) This invention relates to the preparation of phosphine by the electrolysis of phosphorus.

Heretofore, phosphine has ben prepared by the reaction of metallic phosphide or phosphonium halides with water, and by the hydrolysis of elemental phosphorus. These methods have been unsatisfactory because of the high production costs and/ or because the phosphine product is in an impure form.

United States Patent No. 1,375,819, issued April 26, 1921, to Henry Blumenberg, Jr., discloses a method for preparing arsine by the electrolysis of a salt or oxide of arsenic in the presence of sulfuric acid and potassium sulfate or other compounds capable of liberating nascent hydrogen upon electrolysis. However, phosphine is not produced under the conditions set forth by Blumenberg when an oxide or salt of phosphorus is employed.

W. R. Grove, in the Journal of the Chemical Society, vol. 16, (1863), pp. 263-272, discloses the use of an electric current to boil moist molten phosphorus and produces phosphine thereby. Such a technique requires a high voltage, and converts only a small amount of phosphorus to phosphine.

It is an object of this invention to provide a method of producing phosphine by electrolytic means.

Another object of the invention is to provide a more economical method of producing phosphine.

Still another object of the invention is to provide a method of producing phosphine in a form substantiallly free from phosphorus hydrides and other phosphorus impurities.

:Still another object of the invention is to provide a method of producing cathodic phosphine and anodic chlorine simultaneously by electrolytic means.

These and other objects of the invention will be apparent from the following detailed description of the invention.

It has now been discovered that phosphine and chlorine can be prepared by electrolytic means wherein an electric current is passed between an anode and a cathode in contact with an aqueous hydrochloric acid electrolyte, at least a portion of the cathode being in contact with molten phosphorus, while maintaining the anode free from contact with molten phosphorus.

The accompanying drawing is a schematic illustration of a typical electrolytic cell suitable for carrying out the novel process.

Referring to the drawing there is shown cell vessel '10 having a cathode section 11 and an anode section .12, the sections 11 and 12 being separated by a porous dia phragm 13. Gas tight cover 14 having ports :15, 16, 17, 18, and 19, is secured to the top of cell vessel Molten phosphorus is contained in the bottom portion of cathode section 11, the upper level being indicated by interface 21. An aqueous hydrochloric acid electrolyte 22 is contained in the cathode section 1i1 and anode section 12, the upper level of the electrolyte being indicated by electrolyte interface 23. Cathode 24 extends through port 15 and electrolyte 22 into the molten phosphorus 20. Cathode 24 is shown here as a solid plate, but other forms of cathode can be employed, as discussed more fully hereinafter.

Anode 25 extends through port 16 into the electrolyte 22 contained in anode section 12. Electrolytic conductors 26 and 27 connect the anode and cathode, respec- 2 t-ively, to the positive and negative poles, respectively, of a source of electrical energy 28.

When an electric current is impressed upon the system a phosphine-containing gas is generated in the cathode section III and is discharged through catholyte gas discharge line 29, which extends through port 17. At the same time chlorine, the anolyte gas formed in the anode section 12, is discharged through anolyte gas discharge line 31, which extends through port 18.

A fresh supply of molten phosphorus and/or electrolyte may be introduced into cathode section 11 by means of funnel 32 which passes from the cell vessel exterior through port 19 into cathode section 1d. if desired a motor driven impeller '33, or other suitable agitation means, may be positioned in the bottom portion of cathode section ll to effect agitation of the molten phosphorus.

It will be recognized by those skilled in the art that the design of the electrolytic cell shown in the drawing may be modified without departing from the spirit of the invention. For example, the cathode is shown in the drawing as a solid plate, but a liquid cathode such as mercury may be employed. In such a case liquid mercury is placed in the bottom of cathode section 11 below the molten phosphorus, and an electrical conductor 27 is extended through port 15 into the liquid cathode at the bottom of cathode section 1d. In this case it is necessary to agitate both the liquid cathode and the molten phos phorus in order that the liquid cathode contacts not only the molten phosphorus but also the electrolyte, thereby permitting the current to pass between the cathode. and anode.

The cell vessel may be constructed of any impervious material such as glass, ceramics, rubber-lined steel and the like.

Diaphragm 13 may be constructed of any suitable porous material such as sintered glass, porous alundurn, ion-exchange membranes, plastic cloth, glass cloth and the like.

Any material having a hydrogen overvoltage as normally measured in the absence of phosphorus exceeding the hydrogen o'vervoltage of smooth platinum may be employed as the cathode. Typical cathodic materials include lead, lead-mercury amalgam, tin, mercury, cadmium, copper, bismuth, aluminum, zinc, brass, silver, nickel, tellurium, monel, gold, and alloys thereof. For example, the alloy known as Woods metal, which is an alloy containing fifty percent bismuth, twenty-five percent lead, twelve and one-half percent tin, and twelve and onehalf percent cadmium, may be employed. This alloy may be used in either liquid or solid form. Black phosphorus may also be employed as a cathode material.

it is desirable to employ a cathode in a form having a high unit of area per unit of weight. As indicated previously, the drawing shows the cathode in the form of a solid plate. 'If desired, when a solid cathode is employed, the cathode may have the form of a helical coil, wire gauze or screen, perforated sheets, and the like.

The anode is preferably constructed of graphite, but any material capable of resisting anionic corrosion under the electrolysis conditions obtained may be employed. For example, noble metals such as platinum may be employed if desired.

An aqueous hydrochloric acid solution containing between about three and about thirty-seven, and preferably between about fifteen and about twenty-five percent hydrochloric acid by weight, is preferably employed as the electrolyte, but other concentrations can be employed if desired.

Molten white phosphorus, sometimes referred to as yellow phosphorus, is preferably employed as the source of phosphorus for the production of phosphine, but other '2 a allotropic forms of phosphor-us may be employed if desired. The temperature of the phosphorus should be sufficient to maintain it in a molten state, without effecting boiling thereof. For this reason, the temperature of the molten phosphorus and electrolyte is maintained within rthe range between about forty-four degrees and about two hundred and eighty degrees centigrade, and preferably between about rfifty and about one hundred and twenty degrees centigrade. Temperature control of the phosphorus and the electrolyte may be readily obtained by means of a constant temperature bath (not shown in the drawing) surrounding cell vessel 10, but any suitable temperature control means may be employed. For example, on start-up of the electrolytic process, the molten phosphorus and electrolyte may be heated to a temperature within the aforesaid temperature range by means of an external source of heat, and maintained at this temperature by means of a constant temperature bath and/ or the heat generated in the cell.

The rate of phosphine production andthe purity of the phosphine product varies with the current and current density. When a low current density is employed, a gaseous mixture of phosphine and hydrogen is produced at the cathode, the resulting gas mixture containing a high concentration of phosphine. However, under these conditions, the production rate of the gas mixture is relatively low, being generally below the level which is considered economically feasible. When high current densities are employed the production rate of the phosphinecontaining gas is increased, but the concentration of phosphine is reduced. Increasing thecathodic current density will increase the production rate, but also reduces the phosphine concentration in the catholyte gas. It will be recognized by those skilled in the art that the optimum current and optimum current density will vary with the size and design of the cell, but in each case it is important to employ those conditions that yield a catholyte gas having a high concentration of phosphine consistence with commercially feasible production rates. For example, cathodic current density within the range between about rive and about one thousand amperes per square foot yield optimum results in most instances, but higher or lower densities may be employed if desired. It is important to employ, in each case, current which will produce a voltage drop across the system of less than about twenty volts and preferably less than about ten volts. When the voltage drop is in excess of about twenty volts, a significant proportion of the electrical energy imparted to the cell is wasted in heating the ingredients in the cell, rather than effecting electrolytic decomposition thereof.

The catholyte gas produced in accordance with the instant novel process is a mixture of phosphine and hydrogen, containing as bigh as ninety percent phosphine or higher. An important advantage of this process is that the phosphine-containing gas is relatively free of other phosphorus hy-drides compared to phosphine produced by prior art techniques, and as a result the gas is not spontaneously flammable when contacted with air.

The anolyte gas is chlorine gas in relatively pure form.

When hydrochloric acid is used as the electrolyte, the resulting catholyte gas is substantially free from phosphor-us hydride impurities. As a result when this gas is employed as a chemical intermediate, the product of the reaction is in a highly pure form. For example, tetrakis- (hydroxymethyl) phosphonium chloride is prepared by the reaction of phosphine, formaldehyde, and concentrated hydrochloric acid. When phosphine prepared by conventional processes is employed to produce tetrakis- (hydroxymethyl) phosphonium chloride, the resulting product has a purity of about 96.5 percent. In contrast, when phosphine prepared in accordance with the instant novel process is used to prepare tetrakis(hydroxymethyl) phosphoniurnchloride, the purity of the product is as high as 99.9 percent by weight.

'The following examples are presented to define the invention more clearly without any intention of being limited thereby. All parts and percentages are by weight unless otherwise specified.

Example 1 An electrolytic cell was constructed as follows: A five hundred milliliter glass beaker having a gas-tight rubber stopper secured to the top was employed as the cell vessel. A porous sintered glass cylinder was inserted into the cell vessel through the rubber stopper to a point adjacent to the bottom of the cell vessel. The porous sintered glass served as a diaphragm to separate the cathode section (the volume outside of the sintered glass cylinder) from the anode section (the volume inside the sintered glass cylinder).

Fifty milliliters of mercury were placed in the bottom of the cathode section to serve as the cathode. A threeeighths inch diameter graphite rod, which was approximately six inches long, was inserted into the anode section to serve as the anode. Copper wire connected the graphite rod and mercury cathode to a source of direct current. Glass tubing passed through the top of the cell vessel served as a means of removing catholyte gas, and glass tubing inserted in the gas-tight cover of the sintered glass cylinder served to remove the anolyte gas.

Thirty grams of molten phosphorus was placed in the cathode section of the cell vessel. Four hundred milliliters of an aqueous twenty percent hydrochloric acid solution was added to the cathode and anode sections to serve as an electrolyte. The mercury cathode contacted the molten phosphorus as well as the electrolyte. Agitation of the electrolyte, molten phosphorus and mercury was eflected by'means of a plastic coated magnetic stirrer which was rotated at the rate of about two hundred revolutions per minute. I

A current of about three amperes and a voltage of about 4.5 volts were impressed upon the system during electrolysis. The temperature of the electrolyte and molten phosphorus was maintained at about eighty-five degrees centigrade by placing the cell vessel in a constant temperature bath.

The gaseous mixture of phosphine and hydrogen produced at the cathode had a phosphine concentration of 66.5 percent by volume and was produced at the rate of 17.1 milliliters per minute. Substantially pure chlorine was produced at the anode at a corresponding rate.

' It will be recognized by those skilled in the art that various modifications within the invention are possible, some of which are referred to above. Therefore, I do not wish to be limited except as defined by the appended claims.

I claim:

1. The process for the production of phosphine and chlorine which comprises passing an electric current between an anode and a cathode in contact with an aqueous hydrochloric acid electrolyte, said cathode being in contact with molten phosphorus, whereby phosphine is produced at the cathode and whereby chlorine is produced at the anode, and separately recovering the resulting gaseous phosphine and chlorine products.

2. The process of claim 1 wherein said cathode is in agitated contact with said molten phosphorus.

3. The process of claim 1 wherein said cathode is mercury.

4. The process of claim 1 wherein said cathode is a liquid alloy of bismuth, lead, tin and cadmium.

hS. The process of claim 1 wherein said anode is grap ite.

6. The process of claim 1 wherein said aqueous hydrochloric acid electrolyte contains between about three and about thirty-seven percent hydrochloric acid by weight.

7. The process of claim *1 wherein the temperature of the electrolyte and molten phosphorus is maintained during electrolysis within the range between about forty-four and about two hundred and eighty degrees centigrade.

8. The process of claim 1 wherein the temperature of the electrolyte and molten phosphorus is maintained during electrolysis within the range between about fifty and about one hundred and twenty degrees centigrade.

9. The process of claim 1 wherein the current density of said cathode during electrolysis is maintained in the range between about five and about one thousand am- -eres per square foot.

10. The process of claim 9 wherein the voltage during electrolysis is maintained below about twenty volts.

11. The process for preparing phosphine and chlorine which comprises passing an electric current between a graphite anode and a mercury cathode in contact with an aqueous hydrochloric acid electrolyte, a portion of said electrolyte in contact with said mercury cathode being admixed with molten phosphorus, said molten phosphorus :being in agitated contact with said cathode, and maintaining a current density on said mercury cathode of at least about five amperes per square foot, whereby a p'hosp hinecontaining gas is produced at the cathode and whereby a chlorine-containing gas is produced at the anode, and

t3 separately recovering the gaseous phosphine and gaseous chlorine products.

References Qited in the file of this patent UNITED STATES PATENTS 1,375,819 Blurnenberg Apr. 26, 1921 1,970,973 Pahnaer Aug. 21, 1934 2,867,568 Cunningham Jan. 6, 1959 2,913,383 Topter Nov. 17, 1959 FOREIGN PATENTS 1,130,548 France Oct. 1, 1956 OTHER REFERENCES Ephraim: Inorganic Chemistry, 5th edition (1948), pages 617-22.

lauling: College Chemistry (1955), pages 330-5.

Journal of Chemical Society, volume 16 (1863), pages 263-272.

Treatise on Powder Metallurgy by Goetzel, column 111, 1952, page 63 (#907). 

1. THE PROCESS FOR THE PRODUCTION OF PHOSPHINE AND CHLORINE WHICH COMPRISES PASSING AN ELECTRIC CURRENT BETWEEN AN ANODE AND A CATHODE IN CONTACT WITH AN AQUEOUS HYDROCHLORIC ACID ELECTROLYTE, SAID CATHODE BEING IN CONTACT WITH MOLTEN PHOSPHORUS, WHEREBY PHOSPHINE IS PRODUCED AT THE CATHODE AND WHEREBY CHLORINE IS PRODUCED AT THE ANODE, AND SEPARATELY RECOVERING THE RESULTING GASEOUS PHOSPHINE AND CHLORINE PRODUCTS. 